Method and apparatus for reducing smear in back-illuminated imaging sensors

ABSTRACT

A method for fabricating a back-illuminated semiconductor imaging device and resulting imaging device is disclosed, which includes the steps providing a substrate having a front surface and a back surface; growing an epitaxial layer substantially overlying the front surface of the substrate; forming at least one barrier layer substantially within the epitaxial layer; fabricating at least one imaging structure overlying and extending into the epitaxial layer, the imaging structure at least one charge transfer region, the at least one barrier layer substantially underlying the at least one charge transfer region, wherein light incident on the back surface of the substrate generates charge carriers which are diverted away from the at least one charge transfer region by the at least one barrier layer. At least a portion of the epitaxial layer is grown using an epitaxial lateral overgrowth technique. The barrier layer can be a high energy implant formed substantially within the epitaxial layer, an optical shield made of an optically opaque material surrounded by oxide on all sides, or a combination of both. The imaging structure can be a CCD or CMOS imaging structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 60/843,624 filed Sep. 11, 2006, and U.S. provisionalpatent application No. 60/829,947 filed Oct. 18, 2006, the disclosuresof which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The field of the present invention is semiconductor device fabricationand device structure. More specifically, the present invention relatesto a method for reducing smear in back illuminated imaging sensors.

BACKGROUND OF THE INVENTION

CMOS or CCD image sensors are of interest in a wide variety of sensingand imaging applications in a wide range of fields including consumer,commercial, industrial, and space electronics. Imagers based on chargecoupled devices (CCDs) are currently the most widely utilized. CCDs areemployed either in front or back illuminated configurations. Frontilluminated CCD imagers are more cost effective to manufacture than backilluminated CCD imagers such that front illuminated devices dominate theconsumer imaging market. Front-illuminated imagers, however, havesignificant performance limitations such as low fill factor/lowsensitivity. The problem of low fill factor/low sensitivity is typicallydue to shadowing caused by the presence of opaque metal bus lines, andabsorption by an array circuitry structure formed on the front surfacein the pixel region of a front-illuminated imager. Thus, the activeregion of a pixel is typically relatively small (low fill factor) inlarge format (high-resolution) front-illuminated imagers.

An effect called smear can occur when light incident on the front-sideof a front-illuminated imager falls on its charge transfer regionregions, resulting in unwanted generation of charge carriers thatproduce a noise signal in a resulting image. Referring now to FIG. 1, atechnique for reducing or eliminating smear in a typicalfront-illuminated imager is depicted. The front-illuminated imager 2includes a silicon substrate 4, with overlying and alternating lightdetection regions 6 and charge transfer regions 8, separated by pixelisolation implants 10. A CCD gate contact 12 overlies each of the chargetransfer regions 8. To reduce smear, the charge transfer regions areprotected from incident light by placing metal shields 14 over the CCDgate contacts 12 and the charge transfer regions 8. Light falling on theCCD regions can also be made to converge on the detection regions 6 byemploying micro lens (not shown).

Back-illuminated semiconductor imaging devices are advantageous overfront-illuminated imagers for high fill factor, better overallefficiency of charge carrier generation and collection, and are suitablefor small pixel arrays. One goal of the performance of back illuminated,semiconductor imaging devices is that charge carriers generated by lightincident on the back side should be driven to the front side quickly toavoid any horizontal drift, which may smear the image.

Accordingly, what would be desirable, but has not yet been provided, isa device and method for fabricating back illuminated imagers whichincorporate some form of light shielding is needed to avoid smear. Inthe case of back-illuminated imagers, any such shielding should beunderneath the charge transfer regions and also should be well alignedto them.

SUMMARY OF THE INVENTION

Disclosed is a method and resulting device for fabricating aback-illuminated semiconductor imaging device, comprising the steps ofproviding a substrate having a front surface and a back surface; growingan epitaxial layer substantially overlying the front surface of thesubstrate; forming at least one barrier layer substantially within theepitaxial layer; fabricating at least one imaging structure overlyingand extending into the epitaxial layer, the imaging structure comprisingat least one charge transfer region, the at least one barrier layersubstantially underlying the at least one charge transfer region,wherein light incident on the back surface of the substrate generatescharge carriers which are diverted away from the at least one chargetransfer region by the at least one barrier layer. At least a portion ofthe epitaxial layer is grown using an epitaxial lateral overgrowthtechnique.

The resulting back-illuminated semiconductor imaging device, comprises asubstrate having a front surface and a back surface; an epitaxial layersubstantially overlying the front surface of the substrate; at least onebarrier layer substantially formed within the epitaxial layer; at leastone imaging structure formed at least partially overlying and extendinginto said epitaxial layer, the imaging structure comprising at least onecharge transfer region, the at least one barrier layer substantiallyunderlying the at least one charge transfer region; wherein lightincident on the back surface of the substrate generates charge carrierswhich are diverted away from the at least one charge transfer region bythe at least one barrier layer.

The barrier layer can be a high energy implant formed substantiallywithin the epitaxial layer, an optical shield made of an opticallyopaque material surrounded by oxide on all sides, or a combination ofboth. When both a high energy implant and an optical shield areemployed, the optical shield can overly the high energy implant and viceversa. In either case, the high energy implant and the optical shieldare substantially aligned with the overlying charge transfer region ofthe imaging structure. The imaging structure can be, for example, a CCDor CMOS imaging structure.

SUMMARY DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a technique for reducing or eliminating smear in a typicalfront-illuminated imager of the prior art by means of placing metalshields over charge transfer regions;

FIG. 2 depicts a back-illuminated imager employing high energy implants,constructed in accordance with a first embodiment of the presentinvention,

FIG. 3 shows the step of forming alignment keys in the epitaxial layer,according to the first embodiment of the present invention;

FIG. 4 shows a silicon-on-insulator (SOI) substrate used as the startingmaterial for the manufacture of a back-illuminated imager employingoptical shields to reduce smear, constructed in accordance with a secondembodiment of the present invention;

FIG. 5 shows the step of depositing a partial epitaxial layersubstantially overlying a seed layer of the substrate depicted in FIG.4;

FIG. 6 shows the step of depositing a thin oxide layer substantiallyoverlying the partial epitaxial layer of FIG. 5;

FIG. 7 shows the step of patterning layers of oxide and an opticallyopaque layer using photolithography followed by anisotropic etching soas to produce sandwich structures with open gaps therebetween;

FIG. 8 shows the step of filling the open gaps with another layer ofoxide;

FIG. 9 shows a second step of patterning involving photolithography andanisotropic etching to produce an optically opaque layer encapsulated byoxide on the sides, top and bottom, with the remaining surfaces of thepartial epitaxial layer exposed;

FIG. 10 shows the step of using an Epitaxial Lateral Overgrowth (ELO) ora Merged ELO technique to grow a remaining portion of the partialepitaxial layer;

FIG. 11 shows the step of fabricating one or more imaging structures onthe epitaxial layer;

FIG. 12 shows the step of thinning the handle wafer by etching from theback side surface to produce the final of the back-illuminated imager,constructed in accordance with the second embodiment of the presentinvention;

FIG. 13 shows a silicon-on-insulator (SOI) substrate used as thestarting material for the manufacture of a back-illuminated imageremploying both a high energy implant and an optical shields to reducesmear, the optical shield being located between and aligned with chargetransfer regions and the high energy implants, constructed in accordancewith a third embodiment of the present invention;

FIG. 14 shows the step of depositing a partial epitaxial layersubstantially overlying a seed layer of the substrate depicted in FIG.13;

FIG. 15 shows the step of forming a high energy implant in the partialepitaxial layer of FIG. 14;

FIG. 16 shows the step of depositing a thin oxide layer substantiallyoverlying the partial epitaxial layer and the high energy implant ofFIG. 15;

FIG. 17 shows the step of patterning layers of oxide and an opticallyopaque layer using photolithography followed by anisotropic etching soas to produce sandwich structures with open gaps therebetween in thedevice of FIG. 16;

FIG. 18 shows the step of filling the open gaps with another layer ofoxide in the device of FIG. 17;

FIG. 19 shows a second step of patterning involving photolithography andanisotropic etching to produce an optically opaque layer encapsulated byoxide on the sides, top and bottom, with the remaining surfaces of thepartial epitaxial layer exposed in the device of FIG. 18;

FIG. 20 shows the step of using an Epitaxial Lateral Overgrowth (ELO) ora Merged ELO technique to grow a remaining portion of the partialepitaxial layer in the device of FIG. 19;

FIG. 21 shows the step of fabricating one or more imaging components onthe epitaxial layer in the device of FIG. 20;

FIG. 22 shows the step of thinning the handle wafer by etching from theback side surface to produce the final back-illuminated imager,constructed in accordance with the third embodiment of the presentinvention; and

FIG. 23 illustrates a fourth embodiment of a process for manufacturingsmear-free back-illuminated imagers and a resulting device similar tothe device of FIG. 22, except that high energy implants are insertedbetween optical shields and charge transfer regions.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments are intended as exemplary, and not limiting.In keeping with common practice, figures are not necessarily drawn toscale.

FIG. 2 depicts a back-illuminated imager employing high energy implants,constructed in accordance with a first embodiment of the presentinvention, generally indicated at 20. An initial substrate 22, sometimesreferred to in the art as a semiconductor-on-insulator (SOI) substrate,is composed of handle wafer 24 to provide mechanical support duringprocessing, an insulator layer 26 (which can be, for example, a buriedoxide layer of silicon), and seed layer 28. The handle wafer 24 may be astandard silicon wafer used in fabricating integrated circuits.Alternatively, the handle wafer 24 may be any sufficiently rigidsubstrate composed of a material which is compatible with the steps ofthe method disclosed herein. Insulator layer 26 may comprise an oxide ofsilicon with a thickness of about 1 micrometer. Among other embodiments,the thickness of insulator layer 26 may fall in a range from about 10 μmto about 5000 nm. Seed layer 28 may be comprised of crystalline siliconhaving a thickness from about 5 nanometers to about 100 nanometers.

SOI substrates are available commercially and are manufactured byvarious known methods. In one method, thermal silicon oxide is grown onsilicon wafers. Two such wafers are joined with oxidized faces incontact and raised to a high temperature. In some variations, anelectric potential difference is applied across the two wafers and theoxides. The effect of these treatments is to cause the oxide layers onthe two wafers to flow into each other, forming a monolithic bondbetween the wafers. Once the bonding is complete, the silicon on oneside is lapped and polished to the desired thickness of seed layer 28,while the silicon on the opposite side of the oxide forms handle wafer24. The oxide forms insulator layer 26.

Another method of fabricating a SOI substrates begins with obtaining amore standard semiconductor-on-insulator (SOI) wafer in which the seedlayer 28 has a thickness in the range from about 100 nm to about 1000nm. A thermal oxide is grown on the semiconductor substrate, using knownmethods. As the oxide layer grows, semiconductor material of thesemiconductor substrate is consumed. Then the oxide layer is selectivelyetched off, leaving a thinned semiconductor substrate having a desiredseed layer thickness.

SOI substrates manufactured by an alternative method, known as SmartCut™, are sold by Soitec, S.A.

Seed layer 28 may comprise silicon (Si), Germanium (Ge), SiGe alloy, aIII-V semiconductor, a II-VI semiconductor, or any other semiconductormaterial suitable for the fabrication of optoelectronic devices.

A portion of an epitaxial layer 30 is formed on the seed layer 28, usingseed layer 28 as the template. Depending on the material of seed layer28, epitaxial layer 30 may comprise silicon (Si), Germanium (Ge), SiGealloy, a III-V semiconductor, a II-VI semiconductor, or any othersemiconductor material suitable for the fabrication of optoelectronicdevices. Epitaxial layer 30 may have a thickness from about 1micrometers to about 50 micrometers. The resistivity of the epitaxiallayer 30 can be controlled by controlling the epitaxial growth process.

Referring now to FIG. 3, once epitaxial layer 30 is grown, alignmentkeys 34 are printed on and etched into the epitaxial layer 30. Thealignment keys 34 can be used to align subsequent layers during theimager fabrication process. The use of alignment keys can result inhighly accurate alignment of about 0.1 micrometer or less forsubsequently deposited layers. Using photolithography, key patterns 35are printed on a top portion 36 of the epitaxial layer 30. A trench etchprocess can be used to etch the underlying epitaxial layer 30 below thekey patterns 34 until the etched away silicon is stopped by theunderlying insulator/buried oxide layer 26 using plasma etching. Theopen trenches 38 are then filled with a electrically insulating materialsuch as an oxide of silicon, silicon carbide, silicon nitride, orpoly-silicon.

Referring again to FIG. 2, one or more imaging structures 40, such asbut not limited to CCD or CMOS imaging structures, may be fabricated onthe epitaxial layer 30. The one or more imaging structures 40 mayinclude alternating light detection regions 42 and charge transferregions 44, separated by pixel isolation implants (dicing streets) 46,and gate contacts 48 substantially overlying the charge transfer regions44. To reduce smear, the charge transfer regions are protected fromincident light by placing metal shields 50 over the gate contacts 48.More importantly, a high energy ion implantation can be performed, i.e.high energy implants 52 can be formed within the epitaxial layer 30. Fora p-type substrate, high energy implants 52 can be formed by ionimplanting p-type impurities into an p-type epitaxial layer (andlikewise n-type impurities for an n-type epitaxial layer). Therelationship between the high energy implants to protecting overlyingoptical devices from unwanted charge carriers will be discussedhereinbelow.

The high energy implants 52 can act as a potential barrier to chargecarriers 54 generated by light 56 incident on the back side surface 58of the back-illuminated imager 30 that may otherwise migrate toward thecharge transfer regions 44. Instead, the high energy implants 52 causethe charge carriers 56 to move toward the adjacent light detectionregions 42. This prevents the creation of unwanted smear signals.

CMOS imager arrays using high energy implants 52 to prevent smear may beemployed in an active pixel sensor (APS) architecture. APS imagers havea buffer amplifier and other circuits within a pixel. The embedded highenergy implants 52 may used to prevent light from reaching peripheralcircuits on the CMOS imager which may fall outside of the imaging areabut may be illuminated by back illumination. If light is allowed to fallon these CMOS circuit components, undesirable effects such as loss ofsignal charge, streaking in the image, and excess noise may result.

The high energy implant concept is of particular value when used incombination with back illuminated interline transfer (IT) CCD imagers.In an IT-CCD architecture, vertical and horizontal CCD transferregisters need to be shielded from unwanted light, otherwise a smearsignal is produced which can be seen in an output image. Also, theseregisters can produce a smear signal if photoelectrons enter the CCDregister. High energy implant techniques, combined with EpitaxialLateral Overgrowth techniques (ELO), to be discussed hereinbelow, canreduce or eliminate smear in back-illuminated IT-CCD devices.

FIG. 4-12 illustrate a process for manufacturing smear-freeback-illuminated imagers and a resulting device according to a secondembodiment of the present invention, generally indicated at 70, whereinlike reference numbers indicate like structures to those described inthe first embodiment 30 of FIG. 2. Referring now to FIG. 12, whichdepicts the fully manufactured back-illuminated imager 70, opticalshields 72 are deposited and patterned on a partially grown siliconepitaxial layer 30 using an ELO technique. The optical shields 72 arealigned with charge transfer regions 44 during device fabrication.

The optical shields 72 can be made from any suitable opaque materialsuch that light does not penetrate the optical shields 72. In someembodiments, the optical shields 72 can be made from an opticallyabsorptive material. In other embodiments of the present invention, theoptical shields 72 can be made from any suitable reflective material.Suitable opaque materials include high melting point refractory metalssuch as tungsten and its silicides, silicon nitride, silicon carbide,and metallic oxides such as hafnium oxide or zirconium oxides. When anabsorptive material is used, incident light 56 is scattered within theoptical shield 72 or otherwise does not penetrate to the charge transferregions 44, where it can generate unwanted charge carriers. When areflective material is used, light reflected from the optical shields 72is reflected back toward the back side surface 66 of theback-illuminated imager 70, where it may pass a second time through arear portion 73 of the epitaxial layer 30. This is useful for longerwavelengths of the incident light 56, where the absorption length isgreater than the spacing from the optical shields 72 to the back sidesurface 66. This causes more light 56 to be absorbed that generates moreelectron-hole pairs that are collected in the light detection regions42, thereby increasing quantum efficiency.

A doped graded layer 74 formed within the light input side 76 of theoptical shields 72 can reduce recombination of signal charge at theoptical shields 72. The graded doping profile is designed to repel thecharge carriers 54 away from the back surface 66. Wavelengths of lightwith absorption lengths less then the distance from the illuminated backside surface 66 to the optical shields 72 can photo-generatehole-electron pairs with relatively little light reaching the opticalshields 72. As a result, a nearly 100 percent optical fill factor ismaintained. It should be noted that the same optical shields 72 may beused for imagers that use hole detection rather than electron detection.

Referring now to FIG. 4, an SOI substrate 22 is used as the startingmaterial. As with the embodiment of FIG. 2, the SOI substrate 22 iscomposed of handle wafer 24, an insulator layer 26, and seed layer 28.Referring now to FIG. 5, a partial epitaxial layer 78 of about 5 um inthickness with a desired resistivity is grown substantially overlyingthe seed layer 28. Referring now to FIG. 6, a thin oxide layer 80,preferably an oxide of silicon, is deposited substantially overlying thepartial epitaxial layer 78. A suitable optically opaque layer 82(absorptive or reflective) of appropriate thickness is depositedsubstantially overlying the thin oxide layer 80. An additional layer ofoxide 84 is deposited substantially overlying the optically opaque layer82. Referring now to FIG. 7, the layers of oxide 80, 84 and opticallyopaque layer 82 are patterned with photolithography followed byanisotropic etching so as to produce sandwich structures 86 with opengaps 88 therebetween. Alignment keys (not shown) are also printed on andetched into the exposed partial epitaxial layer 78 in a manner similarto that described in FIG. 3 for subsequent optical component alignment.Referring now to FIG. 8, another layer of oxide 90 is deposited in theopen gaps 88, followed by photo and anisotropic etching to produce thedevice configuration of FIG. 9, in which the optically opaque layer 82is encapsulated by oxide on the sides 92, top 94, and bottom 96, withthe remaining surfaces 98 of the partial epitaxial layer 78 exposed.

Referring now to FIG. 10, using an Epitaxial Lateral Overgrowth (ELO) orMerged ELO technique, a remaining portion 100 of the exposed partialepitaxial layer 78 is grown for about 5 um to fill in the open gaps 88and to cover the top oxide portions 94 encapsulating the opticallyopaque layers 82 to form the optical shields 72. Suitable ELO and MELOtechniques for growing the remaining portion 100 of the exposed partialepitaxial layer 78 can be found in Liu, S.; Fechner, P.; Friedrick, J.;Neudeck, G.; Velo, L.; Bousse, L.; Plummer, J., “Structures by selectiveepitaxial lateral overgrowth,” SOS/SOI Technology Workshop, 1988,Proceedings, 1988 IEEE 3-5 Oct. 1988 Page(s): 16, and in Pak, J. J.;Neudeck, G. W.; Kabir, A. E.; DeRoo, D. W.; Staller, S. E., “A newmethod of forming a thin single-crystal silicon diaphragm using mergedepitaxial lateral overgrowth for sensor applications,” Electron DeviceLetters, IEEE Volume 12, Issue 11, November 1991 Page(s): 614-616,respectively, which are incorporated herein by reference in theirentirety. The ELO process is a well studied technique in which amonocrystalline silicon region, obtained by etching openings in athermally grown oxide layer, acts as a seed for single-crystal silicongrown first within the openings and subsequently laterally over theoxide 92, 94, 96. There are also reports on CMOS devices that have beenfabricated on an ELO grown silicon layer, which can be found in Shahidi,G.; Davari, B.; Taur, Y.; Warnock, J.; Wordeman, M. R.; McFarland, P.,Mader, S.; Rodriguez, M.; Assenza, R.; Bronner, G.; Ginsberg, B.; Lii,T.; Polcari, M.; Ning, T. H.; “Fabrication of CMOS on ultrathin SOIobtained by epitaxial lateral overgrowth and chemical-mechanicalpolishing,” Electron Devices Meeting, 1990, Technical Digest.,International 9-12 Dec. 1990 Page(s): 587-590, and in Dolny, G.; Ipri,A.; Jastrzebski, L.; Flatley, D., “Fabrication of SOI CMOS overlarge-area continuous-oxide films formed using epitaxial lateralovergrowth techniques,” Electron Devices, IEEE Transactions on Volume35, Issue 12, December 1988 Page(s): 2437, which are incorporated hereinby reference in their entirety.

Referring now to FIG. 11, after the full epitaxial layer 30 is grown,one or more imaging structure 102 (CCD/CMOS imaging structure) of thetypes described for the first embodiment may be fabricated substantiallyoverlying and extending into the full epitaxial layer 30. The one ormore imaging structures 102 may be aligned on the epitaxial layer 30using alignment keys as a guide so as to substantially overlying theoptical shields 72 using known methods of semiconductor fabrication. Theone or more imaging structures 102 may include alternating lightdetection regions 42 and charge transfer regions 44, as alreadydescribed for the imaging structures of FIG. 2.

After fabrication of the one or more imaging structures 102 iscompleted, the handle wafer 24 is removed by etching from the back sideof the back-illuminated imager 103 as shown in FIG. 12. The insulatorlayer 26 can be thinned to a desired thickness such that it acts as ananti-reflective layer to a desired incoming wavelength of light.Alternatively, the insulating layer 26 can also be removed completely,and another suitable material can be deposited on the remainingepitaxial layer 30 which can be of a desired thickness so as to act asan anti-reflective coating/layer. A similar anti-reflectivecoating/layer technique can be applied to the first embodiment of FIG.2.

FIGS. 13-22 illustrate a third embodiment of a process for manufacturingsmear-free back-illuminated imagers and a resulting device generallyindicated at 110, wherein like reference numbers indicate likestructures to those described in the first embodiment 30 of FIG. 2 andthe second embodiment of FIG. 12. Referring now to FIG. 22, whichdepicts the fully manufactured back-illuminated imager 110, opticalshields 72 are deposited and patterned on a partially grown siliconepitaxial layer 30 using an ELO technique, combined with underlying highenergy implants 52. The optical shields 72 and high energy implants 52are aligned with each other and with the charge transfer regions 44during device fabrication.

Referring now to FIG. 13, an SOI/bulk silicon wafer 22 is used as thestarting material. As with the embodiment of FIG. 4, the SOI/bulksilicon wafer 22 is composed of handle wafer 24, an insulator layer 26,and seed layer 28. Referring now to FIG. 14, a partial epitaxial layer78 of about 5 um in thickness with a desired resistivity is grownsubstantially overlying the seed layer 28. Referring now to FIGS. 15 and22, high energy implants 42 are formed in regions of the partialepitaxial layer 78 to be aligned with the optical shields 72 and thecharge transfer regions 44 to be discussed hereinbelow.

Referring now to FIG. 16, a thin oxide layer 80, preferably an oxide ofsilicon, is deposited substantially overlying the partial epitaxiallayer 78 and the high energy implants 42. A suitable optically opaquelayer 82 (absorptive or reflective) of appropriate thickness isdeposited substantially overlying the thin oxide layer 80. An additionallayer of oxide 84 is deposited substantially overlying the opticallyopaque layer 82. Referring now to FIG. 17, the layers of oxide 80, 84and optically opaque layer 82 are patterned with photolithographyfollowed by anisotropic etching so as to produce sandwich structures 86with open gaps 88 therebetween. Alignment keys (not shown) are alsoprinted on and etched into the exposed partial epitaxial layer 78 in amanner similar to that described in FIG. 3 for subsequent opticalcomponent alignment. Referring now to FIG. 18, another layer of oxide 90is deposited in the open gaps 88, followed by photo and anisotropicetching to produce the device configuration of FIG. 19, in which theoptically opaque layer 82 is encapsulated by oxide on the sides 92, top94, and bottom 96, with the remaining surfaces 98 of the partialepitaxial layer 78 exposed.

Referring now to FIG. 20, using an Epitaxial Lateral Overgrowth (ELO) orMerged ELO technique, a remaining portion 100 of the exposed partialepitaxial layer 78 is grown for about 5 um to fill in the open gaps 88and to cover the top oxide portions 94 encapsulating the opticallyopaque layers 82 to form the optical shields 72.

Referring now to FIG. 21, after the full epitaxial layer 30 is grown,one or more imaging structures 102 of the types described for the firstand second embodiments may be fabricated substantially overlying andextending into the full epitaxial layer 30. The one or more one or moreimaging structures 102 may be aligned on the epitaxial layer 30 usingalignment keys as a guide so as to substantially overlying the opticalshields 72 using known methods of semiconductor fabrication. The one ormore imaging structures 102 may include alternating light detectionregions 42 and charge transfer regions 44, as already described for theimaging structures of FIG. 2.

After fabrication of the one or more imaging components 54 is completed,the handle wafer 24 is removed by etching from the back side of theback-illuminated imager 110 as shown in FIG. 22. The insulator layer 26can be thinned to a desired thickness such that it acts as ananti-reflective layer to a desired incoming wavelength of light.Alternatively, the insulating layer 26 can also be removed completely,and another suitable material can be deposited on the remainingepitaxial layer 30 which can be of a desired thickness so as to act asan anti-reflective coating/layer.

FIG. 23 illustrates a fourth embodiment of a process for manufacturingsmear-free back-illuminated imagers and a resulting device generallyindicated at 120, wherein like reference numbers indicate likestructures to those described in the third embodiment 110 of FIG. 21.FIG. 23 depicts the fully manufactured back-illuminated imager 120,optical shields 72 are deposited and patterned on a partially grownsilicon epitaxial layer 30 using an ELO technique, combined withoverlying high energy implants 52. The optical shields 72 and highenergy implants 52 are aligned with each other and with the chargetransfer regions 44 during device fabrication. The device 120 is similarto the device 110, except that the high energy implant 72 is insertedbetween the optical shields 72 and the charge transfer regions 44. Theonly processing step that differs from the steps outlined in FIGS. 13-22is that the high energy implants 52 are formed in the epitaxial layer 30after the full epitaxial layer 30 is grown, but before one or moreimaging structures 102 are fabricated. The high energy implants 52 andthe one or more imaging structures 102 may be aligned on/in theepitaxial layer 30 using alignment keys as a guide.

It is to be understood that the exemplary embodiments are merelyillustrative of the invention and that many variations of theabove-described embodiments may be devised by one skilled in the artwithout departing from the scope of the invention. It is thereforeintended that all such variations be included within the scope of thefollowing claims and their equivalents.

1. A method for fabricating a semiconductor device, comprising the stepsof: providing a substrate having a front surface and a back surface;growing an epitaxial layer substantially overlying the substrate on thefront surface; depositing at least one optical shield substantiallywithin the epitaxial layer; fabricating at least one structure overlyingand extending into the epitaxial layer, the structure comprising atleast one charge transfer region, the at least one optical shieldsubstantially underlying the at least one charge transfer region,wherein light incident on the back surface of the substrate generatescharge carriers which are diverted away from the at least one chargetransfer region by the at least one optical shield.
 2. The method ofclaim 1, wherein at least a portion of the epitaxial layer is grownusing an epitaxial lateral overgrowth technique.
 3. The method of claim1, wherein the at least one structure is one of a CCD imaging structureand a CMOS imaging structure.
 4. The method of claim 1, wherein the stepof depositing at least one optical shield comprises the steps of:depositing an optically opaque material substantially overlying aportion of the epitaxial layer proximal to front surface of thesubstrate; and substantially surrounding the optically opaque materialwith a layer of oxide material.
 5. The method of claim 4, wherein theoptically opaque material is made from one of an optically reflectiveand an optically absorptive material.
 6. The method of claim 5, whereinthe optically opaque material comprises a doped graded layer.
 7. Themethods of claim 6, wherein the step of depositing at least one opticalshield comprises the steps of: depositing a first oxide layersubstantially overlying a first portion of the epitaxial layer proximalto the front surface of the substrate; depositing an optically opaquelayer substantially overlying the first oxide layer; depositing a secondoxide layer substantially overlying the optically opaque layer; applyinga first photolithographic step followed by a first anisotropic etchingstep so as to expose sides in the optically opaque layer with theremaining surfaces of the first portion of the epitaxial layersubstantially exposed; depositing a third layer of layer of oxidesubstantially overlying the exposed surfaces of the first portion of theepitaxial layer and substantially submersing the sides of the opticallyopaque layer; and applying a second photolithographic step and a secondanisotropic etching step to produce an optically opaque layerencapsulated by oxide on the sides, top and bottom, with the remainingsurfaces of the first portion of the epitaxial layer exposed.
 8. Themethod of claim 7, further comprising the step of growing the remainingportion of the epitaxial layer using an epitaxial lateral overgrowthtechnique.
 9. The method of claim 8, further comprising the step offorming a high energy implant substantially within the remaining portionof the epitaxial layer, wherein the high energy implant andsubstantially underlies the at least one charge transfer region andsubstantially overlies the optical shield.
 10. The method of claim 1,wherein the step of depositing at least one optical shield furthercomprises the steps of: forming a high energy implant substantiallywithin the epitaxial layer proximal to the front surface of thesubstrate and substantially aligned with the charge transfer region; anddepositing at least one optical shield substantially within theepitaxial layer and substantially overlying the high energy implant. 11.The method of claim 10, wherein the step of depositing at least oneoptical shield further comprises the steps of: depositing an opticallyopaque material substantially overlying a first portion of the epitaxiallayer proximal to the front surface of the substrate; and substantiallysurrounding the optically opaque material with a layer of oxidematerial.
 12. The method of claim 11, wherein the of depositing at leastone optical shield further comprises the steps of comprises the stepsof: depositing a first oxide layer substantially overlying a firstportion of the epitaxial layer proximal to the front surface of thesubstrate; depositing an optically opaque layer substantially overlyingthe first oxide layer; depositing a second oxide layer substantiallyoverlying the optically opaque layer; applying a photolithographic stepfollowed by a first anisotropic etching step so as to expose sides inthe optically opaque layer with the remaining surfaces of the firstportion of the epitaxial layer substantially exposed; depositing a thirdlayer of layer of oxide substantially overlying the exposed surfaces ofthe first portion of the epitaxial layer and substantially submersingthe sides of the optically opaque layer; and applying a secondphotolithographic step and a second anisotropic etching step to producean optically opaque layer encapsulated by oxide on the sides, top andbottom, with the remaining surfaces of the first portion of theepitaxial layer exposed.
 13. The method of claim 12, further comprisingthe step of growing the remaining portion of the epitaxial layer usingan epitaxial lateral overgrowth technique.
 14. The method of claim 1,wherein the substrate comprises: a handle wafer; an insulator layersubstantially overlying the handle wafer; and a seed layer substantiallyoverlying the insulator layer; and further comprising the steps ofremoving the handle wafer and thinning the insulating layer such thatthe insulating layer acts as an anti reflective coating.
 15. A device,comprising: a substrate having a front surface and a back surface; anepitaxial layer substantially overlying the front surface of thesubstrate; at least one optical shield substantially formed within theepitaxial layer; at least one structure formed at least partiallyoverlying and extending into said epitaxial layer, the at least onestructure comprising at least one charge transfer region, the at leastone optical shield substantially underlying the at least one chargetransfer region; wherein light incident on the back surface of thesubstrate generates charge carriers which are diverted away from the atleast one charge transfer region by the at least one optical shield. 16.The device of claim 15, wherein at least a portion of the epitaxiallayer is grown using an epitaxial lateral overgrowth technique.
 17. Thedevice of claim 15, wherein the at least one structure is one of a CCDimaging structure and a CMOS imaging structure.
 18. The device of claim15, wherein the at least one optical shield comprises: an opticallyopaque material; and a layer of oxide material substantially surroundingthe optically opaque material.
 19. The device of claim 18, wherein theoptically opaque material is made from one of an optically reflectiveand an optically absorptive material.
 20. The device of claim 19,wherein the optically opaque material comprises a doped graded layer.21. The device of claim 20, wherein at least a portion of the epitaxiallayer is grown using an epitaxial lateral overgrowth technique.
 22. Thedevice of claim 21, further comprising at least one high energy implantsubstantially overlying the optical shield and substantially underlyingthe at least one charge transfer region.
 23. The device of claim 22,further comprising at least one high energy implant substantiallyunderlying the optical shield.
 24. The device of claim 15, wherein thesubstrate comprises: a handle wafer; an insulator layer substantiallyoverlying the handle wafer; and a seed layer substantially overlying theinsulator layer; wherein the handle wafer is removed and the insulatinglayer is thinned such that the insulating layer acts as anantireflective coating.